The controversy over IM and EM tibial guides has existed in the literature since the very introduction of both techniques (1, 9, 17). Based on a systematic review, the published literature includes nearly twenty original articles on this issue over the last three decades. Of these, 52.6% believed the two techniques had comparable accuracy, 36.8% preferred IM guides, and 10.5% found EM guides more accurate (1). However, not all these studies had sufficient sample size, proper methodology, or reported all the important radiologic or functional outcomes. Accordingly, only six of them were eligible for the meta-analysis by Zeng et al. (1). The results of our trial showed no significant difference between the accuracy of the two techniques in terms of the mean post-op MPTA, JLCA, and VA values. However, the number of MPTA outliers was significantly lower in the IM group. The number of outliers for JLCA and VA showed no significant difference. Both groups demonstrated good functional outcomes with no significant difference.
Three radiologic measures have been so far discussed in the literature to compare the accuracy of tibial alignment following IM and EM techniques. These include the MPTA, VA, and tibial slope angles. The mean values of these measures were not significantly different between both techniques in many of the studies. However, it should be noted that the mean value overlooks the presence of outliers, which might be significant when comparing both techniques, noted only by a few studies.
The MPTA is the angle between the tibial mechanical axis and the articular surface of the tibial component in AP knee projection. Different studies have used various terms for it, such as the frontal tibial component angle (1), coronal tibiofemoral angle (15), and tibial component angle (9). Although the mean MPTA reported by Chin and Reed et al. was not significant between both techniques, the two studies were contradictory in terms of the relative risk of outliers. The EM-to-IM risk ratio of outliers was 0.31 versus 2.35 in the studies of Chin and Reed et al., respectively (9, 16). However, the meta-analysis of the pooled data of both studies (20/76 EM vs. 21/84 IM) showed no significant difference (1). Our study also showed also no significant difference between the mean post-op MPTAs of both techniques, although the difference between the outliers was significant (11/42 EM vs. 4/42 IM, EM-to-IM risk ratio = 2.75), consistent with the results of Reed et al. study.
The VA or MFMTA is the angle between the mechanical axis of femur and tibia in AP radiograph, which has also been designated as the mechanical axis angle (1, 16, 18). Neither the mean value nor the outlier relative risk of VA was significantly different between the two techniques in the studies of Chin, Blakeney, and Kroon et al. (16, 18, 19). The meta-analysis of pooled data (31/83 EM vs. 30/89 IM) demonstrated the same result (1). Our result (19/42 EM vs. 16/42 IM) is also consistent with the results of these studies. The tibial slope is another angle, which also showed no significant difference in the meta-analysis study of Zeng et al. (1). We did not report this measure. However, we compared post-op JLCA between the two techniques, not reported before, which also showed no significant difference.
In the ancillary analysis of outlier data, we found that the mean weight of the patients was significantly lower in the MPTA outliers than that of the inliers of the EM group, although no such difference was seen in the IM group. However, BMI showed no significant difference between the outliers and inliers of each group. We found no relevant data in the previous studies comparing both techniques. Nevertheless, a recent study by Compton et al. showed that BMI did not influence the postoperative MPTA following TKA using EM guides. They found no difference between the MPTA outlier (defined as outside ± 5°) ratio of patients with a BMI < 35 (2/100) and ≥ 35 (2/62) (20). It was also not significantly different between those with a BMI < 35 (9/34) and ≥ 35 (1/8) in EM group of our study, which is consistent with the findings of Compton et al. However, the significantly lower weights of the patients, who underwent TKA using an EM guide and have a final MPTA outside the normal range, might indicate that thinner legs are associated with a higher chance of error in the appropriate mounting of an EM jig and doing tibial cuts. It might be due to the underestimation of a thinner leg by the surgeon, as he usually expects the obese legs to be a challenge. We think further comparative studies between both techniques are needed, which focus particularly on the length and girth of the leg to determine the effect of these parameters on the final lower extremity alignment.
The functional outcomes following TKA using either IM or EM techniques have been compared only by a few studies. Cashman et al. reported the SF-36 and WOMAC scores in 103 Triathlon TKAs (36 IM vs. 67 EM), which were not significantly different between both techniques at six months (10). Blakeney et al., in their second study in 2014, compared the physical and mental component scores of SF-12 and the Oxford Knee Score (OKS) between 107 patients undergoing TKA using IM, EM, and computer-assisted techniques at a median follow-up of 46 months. None of these measures differed significantly between IM and EM techniques, although OKS adjusted for sex and age showed a difference, close to the significance level, between computer-assisted and conventional techniques (15). In our study, the postoperative values of KSS, fKSS, pain VAS, and the increase in ROM were not significantly different between IM and EM groups at a mean follow-up of 27 ± 2.9 months. However, a postoperative mechanical axis within ± 3° of neutral was associated with a more increase in knee ROM than that outside this range (Fig. 2).